Not quite sure what that would do, maybe I’m misunderstanding you, that seems to just type the angle brackets that are on the layout in Windows mode and nothing in Mac mode

Maybe that’s a keyboard layout thing that only works with the UK layout then. If I hit AltGr+[ – maybe I’m misremembering, it could be ] – then e.g. a, (using a UK layout) I get ä.

As a backup, when Fedora updates to Plasma 6.7 (probably with Fedora 45) you’ll have the option of long-pressing keys to get a small alternative letter-popup, which of course still adds a keypress so probably also suboptimal.

Several possibilities:

1. As I read the manual Fn+X+F is this keyboard’s equivalent of Fn lock, maybe try that.
2. I believe that AltGr+[ should allow you to type umlauts (at least that works on the GB layout), but I don’t know if that will work on mac mode (I have never used a mac keyboard).
3. Alternatively, you could map the compose key to the print screen key (or some other option you wouldn’t mind losing) and then use that (umlauts should be composed using colon+letter, as far as I recall).